As is known in the art, achieving both high efficiency and high linearity of radio frequency (RF) power amplifier (PA) systems is a longstanding challenge. One means of improving efficiency in such systems is to utilize an architecture in which the PA system is switched with discrete transitions among a set of operating states (e.g., a set of drain bias voltages for one or more power amplifiers in the PA system). This includes systems where the drain voltage of at least one power amplifier is selected (e.g., via switches) from among multiple discrete supply voltages. Discrete output states can also be realized through systems in which the drain voltage is derived from a direct current (DC) voltage source, such as a DC-DC converter that has a plurality of preferred discrete output voltage levels (with the converter output network and/or a filter optionally providing for shaping of transitions between those preferred levels). Other means of synthesizing a set of discrete output levels can likewise be realized.
The nature of the drain voltage transitions in such systems can be important to the RF output quality that is achieved. In particular, the power amplifiers respond to both changes in the RF (gate) input and their DC bias (drain) input.
As is also known, high frequency (e.g., RF) signal components can occur due to changes in a drain voltage and such signal components can be “mixed” (e.g., cross-coupled) with the RF input, yielding undesirable switching signal components in the RF output spectrum around the carrier frequency. However, it is difficult to compensate for such high-frequency drain voltage components via controlling the RF inputs to the PA system. Such undesired components in the RF output might lead to leakage into adjacent channels thereby reducing “linearity” through worsening of “Adjacent Channel Leakage Ratio” (ACLR). Moreover, such undesired components in the RF output can appear in the receive band, contributing to receive-band noise and reducing receiver performance.
Designing multilevel power amplifier systems to mitigate these unwanted spectral components is a challenging task that imposes significant design constraints (e.g., on the range of conditions for which a particular system can operate well.) It is particularly challenging to design such systems to operate well across a wide range of bandwidths and/or in different bands (e.g., with different receive-band spacing and placement) and/or for both time-division duplexing (TDD) and frequency-division duplexing (FDD).